Method for manufacturing porous sheet and porous sheet manufactured by the method

ABSTRACT

The present invention provides a method for producing a porous sheet, including the steps of a) producing a polymer resin sheet containing an object to be processed by supercritical fluid extraction which is dissolved in supercritical fluid; and b) injecting the supercritical fluid into the polymer resin sheet to extract the object to be processed by supercritical fluid extraction that is contained in the polymer resin sheet, thereby forming pores in the polymer resin sheet, and a porous sheet produced by the same.

TECHNICAL FIELD

The preset invention relates to a method for producing a porous sheetusing a supercritical fluid extraction (SFE) method, and a porous sheetproduced by the same.

This application claims priority from Korean Patent Application No.10-2009-0051598 filed on Jun. 10, 2009 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND ART

Chemical-mechanical polishing (CMP) processes are used in themanufacturing of microelectronic devices to form flat surfaces onsemiconductor wafers, field emission displays, and many othermicroelectronic substrates. For example, the manufacture ofsemiconductor devices generally involves the formation of variousprocess layers, selective removal or patterning of portions of thoselayers, and deposition of additional process layers on the surface of asemiconducting substrate to form a semiconductor wafer. The processlayers can include, by way of example, insulation layers, gate oxidelayers, conductive layers, and layers of metal or glass, etc. It isgenerally desirable in certain steps of the wafer process that theuppermost surface of the process layers be planar, i.e., flat, for thedeposition of subsequent layers.

Chemical-mechanical polishing (CMP) processes are used to planarizeprocess layers wherein a deposited material, such as a conductive orinsulating material, is polished and removed to planarize the wafer forsubsequent process steps.

In a typical chemical mechanical polishing (CMP) process, a wafer ismounted upside down on a carrier in a CMP tool. A force pushes thecarrier and the wafer downward toward a polishing pad. The carrier andthe wafer are rotated above the rotating polishing pad on the CMP tool'spolishing table. A polishing composition (also referred to as apolishing slurry) generally is introduced between the rotating wafer andthe rotating polishing pad during the polishing process. The polishingcomposition typically contains a chemical that interacts with ordissolves portions of the uppermost wafer layer(s) and an abrasivematerial that physically removes portions of the layer(s). The wafer andthe polishing pad can be rotated in the same direction or in oppositedirections, whichever is desirable for the particular polishing processbeing carried out. The carrier also can oscillate across the polishingpad on the polishing table.

Polishing pads used in the chemical mechanical polishing (CMP) processare manufactured using both soft and rigid pad materials, which includepolymer-impregnated fabrics, microporous films, cellular polymer foams,non-porous polymer sheets, and sintered thermoplastic particles.

A pad containing a polyurethane resin impregnated into a polyesternon-woven fabric is illustrative of a polymer-impregnated fabricpolishing pad. Microporous polishing pads include microporous urethanefilms coated onto a base material. These polishing pads are closed cell,porous films. Cellular polymer foam polishing pads contain a closed cellstructure that is randomly and uniformly distributed in all threedimensions.

Recently, in most of polishing pads, porous sheets having closed-poresare used in the pad, in which the pores control fluidity of polishingslurry to improve process efficiency. Therefore, it is important touniformly disperse the pores in the polishing pad.

An example of the production method of polishing pads is to produce padsby adding hollow polymeric microelements to a polymeric matrix, asdescribed in Korean Patent No. 10-0191227.

However, the hollow polymeric microelements have shells with a thicknessof several microns, and problematically, these shells generate scratcheson a polishing object, for example, a wafer during chemical mechanicalpolishing (CMP) process.

DISCLOSURE Technical Problem

The present invention provides a method for producing a porous sheet,which forms pores having excellent uniformity and dispersibility,reduces generation of scratches during the process, and improves processefficiency, and a porous sheet produced by the same.

Technical Solution

The present invention provides a method for producing a porous sheet,including the steps of a) producing a polymer resin sheet containing anobject to be processed by supercritical fluid extraction which isdissolved in supercritical fluid; and b) injecting the supercriticalfluid into the polymer resin sheet to extract the object to be processedby supercritical fluid extraction that is contained in the polymer resinsheet, thereby forming pores in the polymer resin sheet.

The present invention provides a porous sheet produced by the productionmethod according to the present invention.

The present invention provides a polishing pad containing the poroussheet according to the present invention.

Advantageous Effects

According to the present invention, provided is a method for producing aporous sheet, capable of forming pores having excellent uniformity anddispersibility in a sheet. In addition, pores can be formed withoutremaining residue in the sheet, because the method is performed byextracting a material dissolved in supercritical fluid. Therefore,during polishing process, generation of scratches on a polishing object,wafer due to residue can be reduced, and the process efficiency can beimproved.

DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the method for producing a porous sheetaccording to the present invention;

FIG. 2 is a SEM photograph of the porous sheet according to Example 1 ofthe present invention;

FIG. 3 is a graph showing the experimental results of Example 1 andExamples 3 to 5; and

FIG. 4 is a SEM photograph of the sheet according to ComparativeExample.

EMBODIMENTS OF THE INVENTION

The method for producing a porous sheet according to the presentinvention includes the steps of a) producing a polymer resin sheetcontaining an object to be processed by supercritical fluid extractionwhich is dissolved in supercritical fluid; and b) injecting thesupercritical fluid into the polymer resin sheet to extract the objectto be processed by supercritical fluid extraction that is contained inthe polymer resin sheet, thereby forming pores in the polymer resinsheet.

In step a), the object to be processed by supercritical fluid extractionmay be one material selected from aromatic compounds, aliphatichydrocarbons and aliphatic alcohols.

Herein, examples of the aromatic compounds may include naphthalene,anthracene, chrysene and pentacene. The aliphatic hydrocarbons may be C₇to C₁₀ aliphatic hydrocarbons, but are not limited thereto, specificexamples thereof may include mineral oil, octane, decane, and dodecane.Examples of the aliphatic alcohols may include heptanol, nonanol anddodecanol.

Among them, it is preferable that the object to be processed bysupercritical fluid extraction may be naphthalene or octane, but is notlimited thereto.

The object to be processed by supercritical fluid extraction that iscontained in the polymer resin sheet may have a round or oval shape, butis not limited thereto.

In step a), the object to be processed by supercritical fluid extractionmay be contained in the polymer resin sheet in an amount of 5 to 50% byweight, and more preferably 20 to 40% by weight.

The polymer resin sheet of step a) may include a polymer resin selectedfrom the group consisting of polyurethane, thermoplastic elastomer,polyolefin, polycarbonate, polyvinyl alcohol, nylon, elastomeric rubber,styrene-based copolymer, polyaromatics, fluoropolymer, polyimide,cross-linked polyurethane, cross-linked polyolefin, polyether,polyester, polyacrylate, elastomeric polyethylene,polytetrafluorethylene, polyethylene terephthalate, polyarylene,polystyrene, polymethylmethacrylate, copolymers thereof, blockcopolymers thereof, mixtures thereof and blends thereof. Herein, thepolyurethane resin is a most preferable material for forming thepolishing pad because it is excellent in abrasion resistance. Inaddition, a polyurethane polymer having desired physical properties canbe easily obtained by changing its raw material composition. Theseproperties of polyurethane are suitable for forming the polishing pad.

Step a) may include the steps of a1) mixing the object to be processedby supercritical fluid extraction with a polymer resin or a precursor;and a2) curing the mixture of step a1).

In step a1), the mixing conditions of the object to be processed bysupercritical fluid extraction and the polymer resin or precursor mayvary depending on impeller speed and reactor temperature in a reactor.In addition, a curing agent may be used during the mixing process,simultaneously. Herein, the impeller speed and reactor temperature maybe controlled variously, but the preferred impeller speed may be 200 to3000 rpm and the preferred reactor temperature may be 40 to 70° C.

In step a1), one or more chain extenders selected from the groupconsisting of 1,4-butanediol, 4,4′-methylenebis(2-chloroaniline),ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol,1,6-hexanediol, neopentyl glycol, 1,4-cyclohexanediol,1,4-cyclohexanedimethanol, 2,2,4-trimethylpentanediol, hydroquinone, bis(2-hydroxyethyl) hydroquinone, 4,4′-dihydroxybiphenyl, bisphenol A,bisphenol F, and mixtures thereof may be further added.

In step a2), the curing process may be performed at 70 to 100° C. for 4to 48 hrs.

In step a), in the case of producing a sheet using a polyurethane resin,the polyurethane may be prepared by an organic polyisocyanate, apolyurethane prepolymer, a polyol compound and a chain extender.

Herein, the organic polyisocyanate of 1 to 20% by weight, thepolyurethane prepolymer of 10 to 88% by weight, the polyol compound of10 to 88% by weight and the chain extender of 1 to 50% by weight may beincluded.

Examples of the organic polyisocyanate may include aromaticdiisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, 2,2′-diphenylmethane diisocyanate, 2,4′-diphenylmethanediisocyanate, 4,4′-diphenylmethane diisocyanate, 1,5-naphthalenediisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate,p-xylylene diisocyanate, and m-xylylene diisocyanate; aliphaticdiisocyanates such as ethylenediisocyanate, 2,2,4-trimethylhexamethylenediisocyanate, and 1,6-hexamethylene diisocyanate; and cycloaliphaticdiisocyanates such as 1,4-cyclohexane diisocyanate,4,4′-dicyclohexylmethane diisocyanate, isophorone diisocyanate,hydrogenated m-xylylene diisocyanate, and norbornane diisocyanate.Mixtures of one or two or more thereof may be used. However, the organicpolyisocyanate is not limited thereto.

The organic polyisocyanate includes not only the diisocyanate compoundsdescribed above but also multifunctional (trifunctional or more)polyisocyanate compounds. As the multifunctional isocyanate compounds,Desmodule-N (manufactured by Bayer Ltd.) or a series of diisocyanateadduct compounds under the trade name of Duranate (Asahi KaseiCorporation) are commercially available.

The polyol compound is exemplified by high-molecular weight polyols suchas polyether polyol, polyester polyol, polycarbonate polyol, and acrylpolyol. In addition to the high-molecular weight polyols, low-molecularweight polyols may be also used. These polyol compounds may be used inmixtures of two or more thereof. However, the polyol compound is notlimited thereto.

The ratio of the organic polyisocyanate, polyol compound and chainextender may vary depending on molecular weight of each compound or useof the polyurethane produced by these compounds (e.g., polishing pad),and desired properties. To obtain a polishing pad having desirablepolishing properties, the number of isocyanate groups of organicpolyisocyanate to total number of functional groups of the polyolcompound and chain extender (the number of active hydrogen groups ofhydroxyl group, amino group, etc.) may preferably range from 0.95 to1.15, and more preferably from 0.99 to 1.10. Meanwhile, the ratio ofhigh molecular weight component to low molecular weight component in thepolyol compound may be determined by the properties required forpolyurethane produced therefrom.

In step a), in the case of producing a sheet using the polyurethaneresin, the polyurethane resin to be a matrix may be produced byapplication of urethane technology such as a melting method, a solutionmethod or the like. In consideration of cost and operating environment,the melting method is preferable.

Any one of one-shot and prepolymer methods may be employed for theproduction of polyurethane. However, in terms of physical properties ofproduced polyurethane, suitable is a prepolymer method, in which anisocyanate-terminal prepolymer from organic polyisocyanate and polyolcompound is previously synthesized and a chain extender is reactedtherewith.

Meanwhile, any commercially available isocyanate-terminal prepolymerthat is produced from organic polyisocyanate and polyol compound may beused, if suitable in the present invention, and also applicable to theproduction of polyurethane by the prepolymer method. Theisocyanate-terminal prepolymer having a molecular weight ofapproximately 800 to 5000 may be suitable in terms of processibility andphysical properties.

In the prepolymer method, the isocyanate-terminal prepolymer is anisocyanate group-containing compound, and the chain extender (ifnecessary, polyol compound) is an active hydrogen group-containingcompound. In the one shot process, the organic polyisocyanate is anisocyanate group-containing compound, and the chain extender and polyolcompound is an active hydrogen group-containing compound.

In the production of polyurethane, stabilizers such as antioxidants,surfactants, lubricants, pigments, fillers, antistatic agents, and otheradditives may be added to the polyurethane stock solution, if necessary.

In step b), the method of injecting supercritical fluid into the polymerresin sheet produced in step a) may be performed by pressurized gasinjection process involving the use of high pressures to forcesupercritical fluid into the polymer resin sheet.

The supercritical fluid injected into the polymer resin sheet in step b)will be described in detail.

Herein, supercritical fluid means a material, having characteristics ofboth liquid and gas at usual temperature and pressure, being at acritical state above supercritical point, that is, at which a liquidcannot be distinguished from a gas at high temperature and pressurecalled a supercritical point, because a chemical can no longer bevaporized.

The supercritical fluid is produced by applying increasing temperatureand pressure to gas, the temperature and pressure being sufficient tomaintain the fluid in a supercritical state.

The gas may be hydrocarbon, chlorofluorocarbon, hydrochlorofluorocarbon(e.g., Freon), nitrogen, carbon dioxide, carbon monoxide or combinationsthereof.

The preferred gas is nonflammable gas, for example, gas having no C—Hbonds. More preferably, the gas is nitrogen, carbon dioxide orcombinations thereof. Most preferably, the gas is carbon dioxide or gascontaining carbon dioxide.

It is preferable that the gas is converted into supercritical gas beforeinjection into the polymer resin sheet.

If the gas is carbon dioxide, the temperature is over 31° C. and thetemperature ranges from 7 MPa (about 1000 psi) to 35 MPa (about 5000psi) (e.g., 19 MPa (about 2800 psi) to 26 MPa (about 3800 psi)).

The supercritical fluid of step b) may preferably include one or moreselected from supercritical carbon dioxide, supercritical isobutane,supercritical butane, supercritical propane, supercritical pentane, andsupercritical nitrogen.

Step b) may be performed at a pressure of 50 to 300 atm and at atemperature of 25 to 120° C., and preferably at a pressure of 70 to 200atm and at a temperature of 30 to 80° C. Step b) may be performed in asupercritical equipment known in the art.

In step b), a supercritical fluid extraction method may be performed bymixing with acetone, alcohol or the like.

Herein, when supercritical fluid is injected, the solvent may be addedat the same time, or the solvent is previously put in a supercriticalreactor, and then mixing process is performed. The solvent may varydepending on the object to be processed by supercritical fluidextraction that is contained the polymer resin sheet. For example,acetone, alcohol, or hexane is a material capable of well dissolving theobject to be processed by supercritical fluid extraction.

As such, when the supercritical fluid of step b) is injected into thepolymer resin sheet produced in step a), the supercritical fluiddissolves the object to be processed by supercritical fluid extractionthat is contained in the sheet, thereby forming pores within the polymerresin sheet produced in step a) without residues. Herein, the formedpores may be closed pores, in which the closed pore means a pore thatdoes not connected to other pores.

In step b), the pores formed within the polymer resin sheet may have around or oval shape, but is not limited thereto.

In step b), the pores formed within the polymer resin sheet may have anaverage diameter of 80 micrometers or less, preferably 5 to 50micrometers, and more preferably 10 to 30 micrometers. Herein, theaverage diameter of the pores denotes the mean values of lines, severalof which pass through the center of circles from their circumference.

In step b), the sheet having the pores formed therewithin may have adensity of 0.5 to 1 g/cm³, preferably 0.6 to 1 g/cm³, and morepreferably 0.7 to 0.9 g/cm³.

In step b), the polymer resin sheet having the pores formed therewithinmay have a porosity of 50% or less, preferably a porosity of 10 to 50%,and more preferably a porosity of 20 to 40%.

As such, according to the present invention, pores may be formed withinthe polymer resin sheet without residues (see FIG. 2). However, in theknown high pressure gas foaming methods, when the foaming process isperformed by injecting into the cured, namely, cross-linkedpolyurethane, there is a problem that foaming may not occur depending onthe degree of cure. In polyurethane being not cross-linked or having alow modulus, foaming occurs by pressurized gas injection, butpractically, it is difficult to show properties suitable for CMP pad.When the foaming process is performed in polyurethane having a highdegree of cure by pressurized gas method, foaming does not occur or apolymeric matrix can be broken (see FIG. 4).

Meanwhile, the present invention provides a porous sheet produced by theabove described method.

The porous sheet according to the present invention may be used as apolishing pad. The porous sheet may be used as a polishing pad, singlyand a plurality of the porous sheets are laminated and used as apolishing pad. In addition, other film attached to the porous sheet maybe used as a polishing pad.

Hereinbelow, the present invention will be described in detail withreference to FIG. 1.

As shown in FIG. 1, the prepolymers of polyurethane are mixed withaliphatic hydrocarbon, naphthalene or fish oil which is dissolved insupercritical fluid, and well dispersed. This prepolymer mixed solutionis reacted with 1.4-butanediol or 4,4′-methylenebis(2-chloroaniline) asa chain extender to form polymer chains. The resultant is cured in a100° C. oven for 4 hrs to 6 hrs in order to form a desired shape througha cast. The cured polyurethane is put in a supercritical equipment toextract a solute. Herein, CO₂ may be used as a supercritical fluid, andsupercritical fluid extraction may be performed by mixing with acetoneor alcohol. Specifically, a material such as acetone or alcohol is putinto a polyurethane sheet to be extracted, simultaneously. That is, asuitable amount of acetone or alcohol is put into the supercriticalfluid extraction equipment, and then the supercritical fluid extractionequipment is closed, followed by injection of CO₂ to a desired pressure.Alternatively, injection of CO₂ into the supercritical equipment isperformed at the same time.

EXAMPLES

Hereinafter, the present invention will be described in more detail withreference to Examples.

Example 1

In Example 1, prepolymer and octane were first mixed with each other toprepare a mixed solution. L325 (manufactured by Chemtura, NCO % 9.17%)was used as a prepolymer, and octane was used as an object to beprocessed by supercritical fluid extraction which is dissolved insupercritical fluid, that is, as a porogen.

First, 1000 g of prepolymer and 400 g of octane was put in a 50° C.reactor, and mixed with each other from 5 min. In this case, octane wasdispersed in prepolymers to form spherical liquid-drops within theprepolymers. Thereafter, 260 g of MOCA (methylenebis(2-chloroaniline))was added thereto, and then mixed. The prepared polyurethane mixture waspoured into a cast, and gelated at a room temperature for 1 hr, and thencured in a 100° C. oven for 24 hrs.

The cured polyurethane mixture was cut in a thickness of 3 mm, andplaced in the supercritical fluid extraction equipment. Temperature ofthe supercritical fluid extraction equipment was set at 45° C., andcarbon dioxide was pressurized into the equipment, and the pressure wasmaintained at 150 bar. The mixture was maintained in the equipment for 1hr, and then depressurized. After the polyurethane sample was taken outof the equipment, and left at room temperature for 1 hr, and then placedin 100° C. oven for 1 hr.

The prepared sample had a density of 0.802 g/cm³ and a Shore D hardnessof 50. A SEM photograph of the sample is shown in FIG. 2. The storagemodulus and tan delta of the sample are shown in FIG. 3.

As such, when a porous sheet is produced by supercritical fluidextraction (SFE) according to the present invention, pores can be formedwithout remaining residue in the sheet because the method is performedby extracting a material dissolved in supercritical fluid. Therefore,generation of scratches on a polishing object due to residue of thesheet during polishing process can be reduced, and the processefficiency can be improved.

Example 2

In Example 2, prepolymer and MOCA were first mixed, and then octane wasmixed therewith.

First, as a prepolymer, 1000 g of L325 (manufactured by Chemtura, NCO %9.17%) was put in a reactor and the temperature was maintained at 50° C.260 g of MOCA previously dissolved was put and mixed at 1000 rpm for 30sec, and then 400 g of octane was mixed therewith.

The prepared polyurethane mixture was poured into a cast, and gelated ata room temperature for 30 min, and then cured in a 100° C. oven for 16hrs.

The cured polyurethane mixture was cut in a thickness of 3 mm, andplaced in the supercritical fluid extraction equipment. Temperature ofthe supercritical fluid extraction equipment was set at 40° C., andcarbon dioxide was pressurized into the equipment, and the pressure wasmaintained at 100 bar. The mixture was maintained in the equipment for 2hrs, and then depressurized. After the polyurethane sample was taken outof the equipment, and left at room temperature for 1 hr, and then placedin 100° C. oven for 1 hr.

Example 3

In Example 3, in order to produce a pad having high modulus, H₁₂MDI wasadded.

The experiment was performed in the same manner as in Example 1, exceptthat H₁₂MDI was added to L325 to prepare prepolymers of NCO % 9.7%. Thestorage modulus and tan delta of the pad produced in Example 3 are shownin FIG. 3.

Example 4

In Example 4, in order to produce a pad having high modulus, H₁₂MDI wasadded.

The experiment was performed in the same manner as in Example 1, exceptthat H₁₂MDI was added to L325 to prepare prepolymers of NCO % 11%. Thestorage modulus and tan delta of the pad produced in Example 4 are shownin FIG. 3.

Example 5

In Example 5, in order to produce a pad having high modulus, H₁₂MDI wasadded.

The experiment was performed in the same manner as in Example 1, exceptthat H₁₂MDI was added to L325 to prepare prepolymers of NCO % 12%. Thestorage modulus and tan delta of the pad produced in Example 5 are shownin FIG. 3.

Example 6

First, 1000 g of prepolymer, 300 g of octane, and 100 g of dodecane wereput in a 50° C. reactor, and mixed with each other for 2 min. In thiscase, octane and dodecane were dispersed in prepolymers to formspherical liquid-drops (1 to 100 micrometers) within the prepolymers.Thereafter, 260 g of MOCA (methylenebis(2-chloroaniline)) was addedthereto, and then mixed. The prepared polyurethane mixture was pouredinto a cast, and gelated at a room temperature for 1 hr, and then curedin a 100° C. oven for 24 hrs. After curing process, the experiment wasperformed in the same manner as in Example 1. A pore of the produced padhad a diameter of 10 to 70 micrometers.

Example 7

In the same manner as in Example 1, the cured pad sheet was put in thesupercritical fluid extraction equipment. The temperature and pressurewere maintained at 50° C. and 150 bar, respectively. After 1 hr, thepressure was eliminated, and the sheet was taken out, and CO₂ wascompletely removed therefrom in a 100° C. oven. The produced pad had adensity of 0.75 g/cm³.

The experimental results of Example 1 and Examples 3 to 5 (storagemodulus and tan delta measured) are shown in FIG. 3, and the measurementmethod of storage modulus and tan delta of FIG. 3 will be described indetail. The storage modulus and tan delta were measured using DMA8000(manufactured by PerkinElmer) at a frequency of 1.5 Hz and amplitude of0.05 mm, scanned at the temperature from -−0° C. to 100° C.

As shown in FIG. 3, the storage modulus of the sample (NCO % 9.17%)produced in Example 1 was found to have 396.5 MPa at 25° C.

The storage modulus of the sample, of which NCO % was increased to 9.7,11, or 12% by addition of H₁₂MDI, was found to have 446.8, 580.3, 698.4MPa, respectively, indicating that the increase of NCO % can improve thestorage modulus.

Meanwhile, as shown in FIG. 3, tan delta tends to reduce as the storagemodulus increases.

Comparative Example

First, as a prepolymer, 1000 g of L325 (manufactured by Chemtura, NCO %9.17%) was put in a 50° C. reactor, and mixed for 5 min. Thereafter, 260g of MOCA (methylenebis(2-chloroaniline)) was added thereto, and thenmixed. The prepared polyurethane mixture was poured into a cast, andgelated at a room temperature for 1 hr, and then cured in a 100° C. ovenfor 24 hrs.

The cured polyurethane mixture was cut in a thickness of 3 mm, andplaced in a supercritical foaming equipment. Temperature of thesupercritical foaming equipment was set at 45° C., and carbon dioxidewas pressurized into the equipment, and the pressure was maintained at150 bar. The mixture was maintained in the equipment for 1 hr, and thendepressurized. After the polyurethane sample was taken out of theequipment, and left at room temperature for 1 hr, and then placed in100° C. oven for 1 hr.

A SEM photograph of the sheet according to Comparative Example wastaken. As a result, cracks were found, as shown in FIG. 4. It was foundthat upon foaming polyurethane having high degree of cure by thepressurized gas method, the polymeric matrix is broken, as shown in FIG.4.

1. A method for producing a porous sheet, comprising the steps of: a)producing a polymer resin sheet containing an object to be processed bysupercritical fluid extraction which is dissolved in supercriticalfluid; and b) injecting the supercritical fluid into the polymer resinsheet to extract the object to be processed by supercritical fluidextraction that is contained in the polymer resin sheet, thereby formingpores in the polymer resin sheet.
 2. The method according to claim 1,wherein the object to be processed by supercritical fluid extractioncontains one or more selected from the group consisting of aromaticcompounds, aliphatic hydrocarbons and aliphatic alcohols.
 3. The methodaccording to claim 1, wherein the polymer resin sheet of step a)contains a polymer resin selected from the group consisting ofpolyurethane, thermoplastic elastomer, polyolefin, polycarbonate,polyvinyl alcohol, nylon, elastomeric rubber, styrene-based copolymer,polyaromatics, fluoropolymer, polyimide, cross-linked polyurethane,cross-linked polyolefin, polyether, polyester, polyacrylate, elastomericpolyethylene, polytetrafluorethylene, polyethylene terephthalate,polyarylene, polystyrene, polymethylmethacrylate, copolymers thereof,block copolymers thereof, mixtures thereof and blends thereof.
 4. Themethod according to claim 1, wherein step a) includes the steps of a1)mixing the object to be processed by supercritical fluid extraction witha polymer resin or a precursor; and a2) curing the mixture of step a1).5. The method according to claim 4, wherein in step a1), one or morechain extenders selected from the group consisting of 1,4-butanediol,4,4′-methylenebis(2-chloroaniline), ethylene glycol, 1,2-propanediol,1,3-propanediol, 1,2-butanediol, 1,6-hexanediol, neopentyl glycol,1,4-cyclohexanediol, 1,4-cyclohexanedimethanol,2,2,4-trimethylpentanediol, hydroquinone, bis (2-hydroxyethyl)hydroquinone, 4,4′-dihydroxybiphenyl, bisphenol A, bisphenol F, andmixtures thereof are further added.
 6. The method according to claim 4,wherein in step a2), the curing process is performed at 70 to 100° C.for 4 to 48 hrs.
 7. The method according to claim 1, wherein in step a),the object to be processed by supercritical fluid extraction iscontained in the polymer resin sheet in an amount of 5 to 50% by weight.8. The method according to claim 1, wherein the supercritical fluid ofstep b) contains one or more selected from the consisting ofsupercritical carbon dioxide, supercritical isobutane, supercriticalbutane, supercritical propane, supercritical pentane, and supercriticalnitrogen.
 9. The method according to claim 1, wherein the step b) isperformed at a pressure of 50 to 300 atm and a temperature of 25 to 120°C.
 10. The method according to claim 1, wherein in step b), the poresformed within the polymer resin sheet have an average diameter of 80micrometers or less.
 11. The method according to claim 1, wherein instep b), the sheet having the pores formed within the polymer resinsheet may have a density of 0.5 to 1 g/cm3.
 12. The method according toclaim 1, wherein in step b), the polymer resin sheet having the poreshas a porosity of 50% or less.
 13. A porous sheet produced by the methodaccording to claim
 1. 14. A porous sheet having pores, which is formedby extraction of an object to be processed by supercritical fluidextraction contained in a polymer resin sheet.
 15. The porous sheetaccording to claim 14, wherein the object to be processed bysupercritical fluid extraction contains one or more selected from theconsisting of aromatic compounds, aliphatic hydrocarbons and aliphaticalcohols.
 16. A polishing pad including the porous sheet according toclaim
 13. 17. A polishing pad including the porous sheet according toclaim 14.